# ---
# title: 39. Combination Sum
# id: problem39
# author: Tian Jun
# date: 2020-10-31
# difficulty: Medium
# categories: Array, Backtracking
# link: <https://leetcode.com/problems/combination-sum/description/>
# hidden: true
# ---
# 
# Given an array of **distinct** integers `candidates` and a target integer
# `target`, return _a list of all **unique combinations** of _`candidates`
# _where the chosen numbers sum to_`target` _._ You may return the combinations
# in **any order**.
# 
# The **same** number may be chosen from `candidates` an **unlimited number of
# times**. Two combinations are unique if the frequency of at least one of the
# chosen numbers is different.
# 
# It is **guaranteed** that the number of unique combinations that sum up to
# `target` is less than `150` combinations for the given input.
# 
# 
# 
# **Example 1:**
# 
#     
#     
#     Input: candidates = [2,3,6,7], target = 7
#     Output: [[2,2,3],[7]]
#     Explanation:
#     2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
#     7 is a candidate, and 7 = 7.
#     These are the only two combinations.
#     
# 
# **Example 2:**
# 
#     
#     
#     Input: candidates = [2,3,5], target = 8
#     Output: [[2,2,2,2],[2,3,3],[3,5]]
#     
# 
# **Example 3:**
# 
#     
#     
#     Input: candidates = [2], target = 1
#     Output: []
#     
# 
# **Example 4:**
# 
#     
#     
#     Input: candidates = [1], target = 1
#     Output: [[1]]
#     
# 
# **Example 5:**
# 
#     
#     
#     Input: candidates = [1], target = 2
#     Output: [[1,1]]
#     
# 
# 
# 
# **Constraints:**
# 
#   * `1 <= candidates.length <= 30`
#   * `1 <= candidates[i] <= 200`
#   * All elements of `candidates` are **distinct**.
#   * `1 <= target <= 500`
# 
# 
## @lc code=start
using LeetCode

## add your code here:
## @lc code=end
